It is proposed to investigate the role nicotinamide plays in preventing the embryocidal and teratogenic action of 2-amino-1,3,4-thiadiazole (ATDA) in the pregnant Wistar albino rat. I have already shown that simultaneous intraperitoneal injections of nicotinamide with ATDA on gestation day 11 effectively inhibits the action of ATDA. The first series of experiments will consist of administering nicotinamide and ATDA simultaneously throughout the teratogenic period (day 5 to day 15) in order to determine the effect of nicotinamide on gestation days other than day 11. Subsequently, on day 11, the time interval between the injection of the teratogen and the injection of nicotinamide will be increased to determine the effective period of action of nicotinamide. As nicotinamide is principally converted in the body to the coenzyme nicotinamide adenine dinucleotide (NAD), the activity of NAD will be measured in embryonic tissue to determine whether or not ATDA acts to suppress the synthesis or utilization of NAD. It is proposed to investigate the influence of teratogens known to cause specific limb defects (carbonic anhydrase inhibitors and ATDA) on acid mucopolysaccharide synthesis in developing embryonic limbs. A detailed analysis will be made by daily observation of the normal synthesis of acid mucopolysaccharides in the embryonic limb from an early limb bud stage (gestation day 13) until late in the gestation period (day 20). The rate of uptake of S35-sulfate will be used as a measure of the rate of mucopolysaccharide synthesis. Embryonic limbs will be incubated in vitro in the presence of S35-labeled sodium sulfate and the rate of sulfate incorporation determined by the method of liquid scintillation counting. The normal rate of synthesis will be compared with the rate of synthesis from embryos whose mothers have been treated with the teratogens.